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Speed limit found for sluggish dark matter

By Marcus Woo

Clues everywhere, if you know how to look

(Image: Marusa Bradac/SLAC)

Step aside, speed of light. Now we know the speed of dark.

If particles of dark matter had never formed the clumps they are in today, they would scurry around space at no more than a sluggish 54 metres per second. The finding is one of the few known values for a characteristic of “cold dark matter”, thought to be the most common type of the stuff in the universe.

Based on the motions of stars and galaxies, we know the universe is filled with more mass than we can see. Astronomers have calculated that this invisible dark matter makes up about 80 per cent of the universe’s matter, and that today most of it is bunched up in huge haloes that surround galaxies.

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Although we can see dark matter’s gravitational effects, it does not otherwise interact with ordinary matter, and we know frustratingly little about its properties. But our best theories say that most of it should be “cold”, meaning it is made of particles that move well below the speed of light.

It is thought that as the hot soup of the early universe cooled, free-ranging dark matter particles coalesced, and their gravity started pulling together gas and dust to make the first stars and galaxies. The speed of the dark particles would have influenced how easily they could form clumps, and faster stuff would not have been able to give rise to the large-scale structure we see today.

Cold comfort

Now, for the first time, Cristian Armendáriz-Picón and Jayanth Neelakanta of Syracuse University in New York have calculated how fast unclumped dark matter particles would zip around randomly in space. The team looked at snapshots of how matter was distributed at different points in cosmic history. They examined the distribution at very small scales, given by spectral analysis of the earliest light in the universe, and at much larger scales, given by surveys of galaxy clusters. They calculated how fast dark matter must have been moving in the early universe to create the observed large-scale structure and then extrapolated how fast the particles would be moving today if they were not in clumps.

The newly calculated number is about what would be expected according to most theories of cold dark matter, says Scott Dodelson at the Fermi National Accelerator Laboratory in Batavia, Illinois, who was not involved in the work. Still, having a quantified value is useful, he says. “It goes without saying that we know very little about dark matter, so anything we learn is pretty important,” says Dodelson.

Future work could use the speed to test models of the particles that make up cold dark matter, which should bring us closer to identifying the elusive substance.